This invention relates to modulators, such as filters and choppers, for use with infra-red radiation.
In certain applications of infra-red imagers, there is a need for improved discrimination of the imaged radiation. This requirement could in principle be met by the use of a number of infra-red spectral filters, each covering a respective wavelength range in the overall wavelength band, and substituting filters until acceptable performance was achieved. However, it is not practical to continuously change the spectral filter. It is possible to process each wavelength range in parallel, but this would greatly increase the cost, complexity, weight and volume of the equipment.
It is normal practice in an infra-red imager to interrupt the received infra-red radiation at regular intervals using a chopper comprising rotating blades. However, this involves the use of mechanical movement and of an electric motor, which can cause problems in generating local heat which has to be properly allowed for.
It has previously been proposed to use a compound semiconductor crystal of cadmium telluride doped with iron ions to intensity modulate infra red radiation, as described in U.S. Pat. No. 3,516,728. However, cadmium telluride has, inherently, a very small band gap and hence, hot carriers, which are not in equilibrium with the crystal lattice structure, are caused to transfer their energy to Fe ions, thereby in effect to modify the bandgap, and as a consequence of the new energy state of the Fe ions, the optical absorption of the crystal is changed. Very high voltage levels are required, in the order of 3000 volts to approach 100% relative absorption, and the crystal must be maintained in a low temperature environment. Furthermore, as the hot carriers are not in equilibrium with the lattice, filtering is localised to a relatively small region of the crystal. Additionally, although the relative slope of the band edge may be altered slightly by the application of various values of modulating current the device has a relatively fixed pass band. Hence, the band edge of the device cannot be swept so as to cause the device to wavelength scan incident infra red radiation.
It has also been proposed, in U.S. Pat. No. 4,190,811, to switch a high power signal laser beam having a predetermined infra red radiation frequency by the use of a semiconductor which is transparent to the signal laser beam. A control beam is used to irradiate the surface of the semiconductor, the radiation beam having a frequency sufficiently high to produce free carriers in the semiconductor and sufficient radiation intensity and time duration to produce a free carrier density sufficient to cause reflection of the signal beam. With such devices, however, a relatively intense control beam is required. Furthermore it is relatively difficult to control accurately the density of the free carriers generated by the use of an irradiated beam as absorption of the beam by the semiconductor material is relatively inefficient. Optical control of the free carrier concentration makes such devices unsuitable for many applications.